Metal anodes of valve metal such as titanium or alloys thereof having electrocatalytic coatings of platinum metals, platinum metal oxides, mixtures of valve metal oxides or other oxides with platinum metal oxides, and so-called mixed crystal material for use in the electrolytic alkali chlorate and chlorine cell fields have been of much interest in recent years. In this art, the term "film-forming metal" is also used to refer to the valve metals. The problems of protecting the valve metal base, also known as the anode substrate, of such anodes from attack and damage under electrolysis conditions have also been of interest. Platinum metal coated anodes have been described in U.S. Pat. Nos. 3,177,131 and 3,265,526. Platinum metal oxide coatings have been described in U.S. Pat. Nos. 3,711,385, 3,864,163, Reissue 28,820, and 4,052,271 (or Canadian Patent 932,699). Dutch Patent 6,606,302 of Nov. 14, 1966, discloses platinum metal oxide coatings wherein this material is mixed with non-platinum metal oxide. These publications disclose an electrode and the method of making such an electrode consisting of a core of film-forming metal or alloy thereof, to which is applied a thin coating of platinum metal oxide, and which core of film-forming metal may be in the form of a jacket over a conductive material isolated from the electrolyte. The art further teaches, particularly in respect to titanium as the core of film-forming metal, the creation of a porous oxide layer thereon to promote adhesion of the platinum metal oxide, or the application of the precious metal oxide to porous titanium and then subsequent rolling to reduce the porosity.
Mixtures of a valve metal oxide such as titanium dioxide with a precious metal oxide to form the electrocatalytic coating on a valve metal core or base for use as an anode are described in U.S. Pat. Nos. 3,773,554 and 4,112,140, and variations thereof are elsewhere described, as for example, in Dutch Patent 6,606,302, above cited, wherein the use of a valve metal oxide as the non-platinum metal oxide is described.
So-called mixed crystal material of a platinum metal oxide with a film-forming metal oxide which is described in terms of its behavior in ionic electrical energy conductance in contact with the electrolyte and is applied as a coating to a valve metal base or core and with the film-forming metal oxide comprising more than 50% of the coating, is disclosed in German Patent application 1,671,422, published Oct. 19, 1972. Still other prior art; namely, U.S. Pat. Nos. 3,632,498; 3,751,296; 3,778,307; 3,933,616 (or Canadian Patent 932,700) disclose an electrode and the method of making such electrodes comprising a base of a metal or metal alloy or non-metallic conductor such as graphite upon which is a coating of so-called mixed crystal material comprising 50 mole percent or more of the oxide film-forming metal together with up to 50 mole percent of oxide of a precious metal. This art teaches the means of making such electrode by coprecipitation upon a base of conductive film-forming metal of the same metal as of the film-forming metal oxide. Also taught in this art is the making of the electrode by sputtering techniques and by electro deposition. Coprecipitation of the film-forming metal oxide with the conducting precious metal oxide onto the film-forming base, according to the art, firmly adheres the precious metal oxide to the film-forming substrate in a manner not heretofore possible.
The problems of the electrolysis product attack on the valve metal base or core of such catalytic anode coatings disclosed in the above references is disclosed in U.S. Pat. No. 3,096,272 in which a barrier layer of titanium oxide is formed between the pores of the noble metal coating by high temperature (800.degree.) thermal methods and U.S. Pat. No. 3,236,756 by electrochemical methods. U.S. Pat. No. 3,234,110 discloses an electrode comprising a core of titanium metal or an alloy thereof to which is applied by electrolytic deposition a barrier layer of titanium oxide and over the surface of which is applied a platinum (noble) metal catalytic coating. U.S. Pat. No. 3,773,555 discloses an improved method of applying a barrier layer of film forming metal oxide on a film-forming metal core prior to applying the catalytic coating of a platinum metal or an oxide of a platinum metal.
By "valve metal" or "film-forming metal" is meant a metal or alloy which, when connected as an anode in the electrolyte and under the conditions in which the metal or the alloy is subsequently to operate as an anode, exhibits the phenomenon that within a few seconds the passage of the electrolysis current drops to less than 1% of the original value. For purposes of this invention, examples of these metals are titanium, titanium alloys, tantalum, tantalum alloys, zirconium, zirconium alloys, niobium, and niobium alloys and tungsten and tungsten alloys. Thus, the terms "film-forming metal" and "valve metal" are used herein in accordance with their art recognized meaning.
More recently, it has been thought that at least one of the modes of anode passivation or anode coating failure is the gradual build-up of a non-conducting titanium oxide layer between the applied catalytic coating and the titanium core. See T. Loucka, Journal of Applied Electrochemistry, 1977. This oxide layer would form if, over a period of time, enough oxygen diffuses through the coating and reacts with the titanium underneath the coating to form an insulator over the conductive metal core. The anode passivation can be delayed by applying a thicker precious metal coating, but this is undesirable from an economic point of view. The passivation may also be delayed by providing a conductive layer which acts as a barrier to oxygen diffusion or by providing a non-oxide forming inter-layer. This is also undesirable because of difficulties caused by increased electrode resistance (between layers) as well as adhesion of the outer coating.
Difficulties in the art of making platinum (noble) metal and platinum (noble) metal oxide coated anodes with a satisfactory economic long life are further evidenced by "Modern Chlor-Alkali Technology," Volume 1, (1979) pp. 108-117. Mechanical breakage occurs because of changes in the stress pattern of platinum metal coatings from dissolution of the platinum and gas bubble impingement, and attack on the titanium core through pores in the coating. U.S. Pat. No. 4,140,813 addresses this problem by disclosing the flame or plasma spraying of from 50 to 6,000 grams/m.sup.2 of titanium oxide onto the core or base prior to application of the electrochemically active substance containing a platinum metal or oxide thereof. The functioning of such barrier layers is described in the Journal of Applied Electrochemistry 13 (1983) pp. 341-350 authored by D. Bergner and Katowski.
Other prior developments have proposed intermediate layers in electrode manufacture. For example, Martinson, U.S. Pat. No. 3,711,397 suggests using an intermediate electroconductive layer as a binding agent.
Westerlund, U.S. Pat. No. 4,098,671 proposes an intermediate layer of MoS.sub.2 for a coated cathode. Bouy et al. U.S. Pat. No. 4,222,842 discloses an intermediate layer of oxide or hydride of titanium.
It is, therefore, apparent that difficulties in maintaining in service adhesion of the platinum metal or platinum metal oxides to conductive valve metal cores or bases, and avoidance of passivation thereof and attack by the electrolytic products on the cores or bases through the pores of the platinum metal or platinum metal oxide coatings have been encountered in carrying out prior art methods. As a result, complicated and/or expensive manufacturing or processing procedures have been developed in an attempt to overcome the deficiencies.
As a further indication of this, in U.S. Pat. No. 3,775,284, Bennett and O'Leary teach the use of an electrodeposited layer of platinum, which is subsequently heated at 450.degree. C. Not only is this platinum layer required before the subsequent intermediate barrier layer is applied, but involves both an electrodeposition step as well as a relatively high temperature baking step. They state that "it appears at this time that the heat treatment is critical since it has been found that the solid solution coating will not adhere to the untreated metal itself . . . " (column 4, lines 14-17).
Other prior art which suggests use of an intermediate barrier layer is typified by the use of a relatively higher temperature requirement for the formation of a suitable barrier layer, such as in Canadian Patent 936,836, or in U.S. Pat. No. 3,986,942.
With regard to the use of titanium based anodes and titanium based cathodes, the electrodes or electrode substrate coatings are known not to be generally interchangeable. Titanium metal, being a valve metal, will passivate when polarized anodically; under cathodic polarization, however, the titanium will not passivate, but will continue to pass electrical current, even after titanium hydride forms on the surface under hydrogen evolution conditions. Titanium anodes, therefore, require protective (and catalytic) coatings under anodic, oxidizing conditions.